The invention relates to a method for the production of a high quality free-standing layer of Gallium Nitride or similar material by heteroepitaxial deposition and subsequent removal from a transparent substrate.
Gallium Nitride (GaN) has been recognized as having great potential as a technological material. For example, GaN is used in the manufacture of blue light emitting diodes, semiconductor lasers, and other opto-electronic devices, as well as in the fabrication of high-temperature electronics devices.
One of the greatest challenges for the large-scale production of GaN-based devices is the lack of a suitable native GaN substrate. GaN is not found in nature; it cannot be melted and pulled from a boule like silicon, gallium arsenide, sapphire, etc., because at reasonable pressures its theoretical melting temperature exceeds its dissociation temperature. However, the fabrication of very high crystal quality, thin layers of GaN, and its related alloys, for use in electronic devices, requires that they be deposited homoepitaxially onto an existing GaN surface. Such high quality device layers cannot be directly grown heteroepitaxially, for reasons that are outside the scope of this invention.
The techniques currently in use for the fabrication of high quality GaN and related layers involve the heteroepitaxial deposition of a GaN device layer onto a suitable but non-ideal substrate. Currently such substrates include (but are not limited to) materials such as sapphire, silicon, silicon carbide, gallium arsenide, lithium gallate, lithium aluminate, and lithium aluminum gallate. All heteroepitaxial substrates present challenges to the high-quality deposition of GaN, in the form of lattice and thermal mismatch. Lattice mismatch is caused by the difference in interatomic spacing of atoms in dissimilar crystals. Thermal mismatch is caused by differences in the coefficient of thermal expansion (CTE) between joined dissimilar materials, as the temperature is raised or lowered.
For the purpose of clarity, heteroepitaxial growth is defined herein as a process whereby the atomic lattices of two dissimilar materials are intimately joined together by atomic bonds across their common interface. When the cross-linking bonds are made in a regular and orderly array displaying long-range order, the interface is said to be coherent. When the cross-linking bonds are broken, bent, twisted, or otherwise distorted such that there is no long-range order, the interface is said to have lost coherency. Coherent interfaces are much stronger than incoherent interfaces, due to the greater number of cross-linking bonds between the materials. The loss of coherency may be partial; if only a percentage of cross-linking bonds are broken or distorted in an interface, the interface is partially coherent. The percentage (by area) of broken or distorted bonds represents the level of incoherency or loss of coherency for that interface.
The most commonly used heteroepitaxial substrate for GaN deposition is sapphire (Al2O3), which has both a large thermal mismatch and a large lattice mismatch compared to GaN. In addition, the sapphire substrate is not electrically conductive, and has poor thermal conductivity, limiting its heat sinking capabilities, further reducing device performance and complicating device processing. For reasons unrelated to the scope of this invention, sapphire otherwise possesses superior properties as a hetero-substrate. However, the large lattice mismatch results in films that have very high defect densities, specifically in the form of dislocations, which are especially undesirable from a device fabrication point of view. (The formation of dislocations at regular intervals along the interface does not affect its coherency, as defined for the purposes of this application, for the dislocations themselves exhibit a type of long-range order in their distribution.) As with other epitaxial crystal growth processes, it is necessary to grow a buffer layer of GaN on the sapphire surface prior to the formation of device-quality layers. The buffer layer will vary, depending on device tolerance to dislocations, whether or not special growth techniques (such as growth through a mask pattern, use of low temperature buffer layers, etc.) are employed, as well as other factors. Typically, this GaN buffer thickness is less than one micron to tens of microns thick. Defect densities, however, predominantly in the form of dislocations, remain high (xcx9c1010 cmxe2x88x922) resulting in diminished device quality. In addition to the conventional buffer layer, a low temperature GaN buffer layer is nearly always used. This layer is the first layer deposited on the sapphire. The buffer layer is initially amorphous and typically is 30-50 nm thick; it is recrystallized at the growth temperature.
Besides dislocations and lattice mismatch problems, thermal mismatch is also a consideration. Typically the GaN is deposited onto sapphire at a temperature of between 1000-1100xc2x0 C.; as the sample cools to room temperature, the difference in thermal expansion (contraction) rates gives rise to high levels of stress at the interface between the two materials. Sapphire has a higher coefficient of thermal expansion (CTE) than does GaN. As the sapphire substrate and GaN layer cool, the mismatch at the interface puts the GaN under compression and the sapphire under tension. Up to a point, the amount of stress is directly related to the thickness of the deposited GaN, such that the thicker the film, the greater the stress. Above a film thickness of approximately 10 microns, the stress levels exceed the fracture limits of the GaN, and cracking and peeling of the film may result. Cracks in this layer are much less desirable than high dislocation densities, and should be avoided because of the risk of their catastrophic propagation into the device layer during subsequent processing steps.
One method to prevent such thermal stress-related problems involves separating the sapphire substrate from the deposited film. This may be done by physically removing the substrate (lapping and polishing), or by focusing a very high-intensity light source (such as from a laser) from the substrate side of the sample. The light source emits photons having an emission energy that is not absorbed by the sapphire. This second technique utilizes the difference in absorption between the two materials: GaN has a room temperature electron bandgap of approximately 3.45 eV, whereas sapphire has a bandgap of 9.9 eV. Photons with an energy greater than approximately 3.45 eV and less than 9.9 eV (corresponding to vacuum wavelengths less than 359 nm but greater than 125 nm) are able to pass through the back side of a sapphire wafer, where they are absorbed in various amounts, depending on energy, by the GaN at the interface. Once absorbed, the photons are converted to heat, which locally disrupts the Gaxe2x80x94N bonds. If the incident radiation is intense enough, large-scale local disruption results in a complete loss of coherency between the lattice of the sapphire substrate and the GaN. At lower radiation levels, the loss of coherency may only be partial and incomplete, resulting in a film that is still attached to the sapphire substrate, but is no longer completely bonded to it.
Both aforementioned techniques have limitations. A free-standing film must be sufficiently thick to have the required mechanical strength necessary for subsequent device processing. Typically, this requires a minimum thickness on the order of 50-100 microns. Deposition of a crack-free film with this thickness onto sapphire is feasible if done carefully, however thermal stresses will cause severe bowing in the wafer as it cools to room temperature. Conventional lapping and polishing processes are not effective at removing a concave substrate; alternatively, use of a laser to remove the GaN from the sapphire can create unstable localized regions of stress in the partially-removed film, leading to layer fracture during the lift-off process.
Referring to the drawings, FIGS. 1(a)-1(b) schematically illustrate the prior art when deposition of a thick layer of GaN onto sapphire is desired. In FIG. 1(a), sapphire substrate 101 has a thick (greater than 10 microns) film of GaN 102 deposited onto it, at the growth temperature, which may be in the range of 1000-1100xc2x0 C. The actual method of deposition is not relevant to this invention. Because the film of GaN nucleates onto the substrate at this temperature, there is no thermal stress present. FIG. 1(b) shows the effects of the large temperature change as the sample cools to room temperature. In this figure, sapphire substrate 101 is now under compressive stress and is bent concave with respect to the deposited film. If the stresses are great enough, cracks 103 may form in the substrate. The epitaxial GaN 102 is under tensile stress, and is cracked, and may also peel away from or otherwise degrade the interface with substrate 101.
FIG. 2 schematically represents a series of steps involved in the conventional method for making a thick layer on a thermally and/or lattice mismatched substrate. Step 201 calls for the provision of a prepared substrate. This prepared substrate may be, for example, plain sapphire, chemically cleaned prior to use. Step 202 is the setting of process parameters and growth conditions for the growth of the thick, flat, high quality layer. Typically these conditions are growth temperature, growth rate, flow rates for precursor compounds, and relative ratios of gas flows in the reactor. Step 203 calls for the deposition of the thick layer 102 onto the prepared substrate. The thickness of this layer is preferentially in the range of 10-400 microns. In step 204, the sample is cooled down to room temperature where it is removed, intact, from the reactor. The wafer is bowed due to the residual stress caused by the thermal mismatch between the epitaxial layer and the substrate. This stress also leads to the formation of many cracks 103 in the thick layer and the substrate.
FIGS. 3(a)-3(d) schematically illustrate the prior art technique of using laser lift-off (LLO) to release a deposited GaN film from the sapphire substrate. In FIG. 3(a), sapphire substrate 301 has had a film of GaN 302 deposited onto it, at the growth temperature, and has subsequently been cooled to room temperature. Film 302 is deposited in such a manner that cracks do not form during the cooling down stage. In FIG. 3(b), laser beam 303 impinges upon the back side of the sapphire substrate. The laser is of an energy such that its photons are strongly absorbed by the GaN layer, while passing through the sapphire largely unabsorbed. Typically the energy range of such photons is above 3.45 eV, corresponding to a wavelength (in vacuum) of less than 359 nm but greater than 125 nm. The source of these photons is typically a pulsed ultraviolet laser, such as a tripled YAG or excimer laser; however the characteristics that are important for this process are not laser-specific. Any highly intense light source that can be focused down to a spot will suffice. Because the beam impinges from the sapphire side of the wafer, the GaN at the sapphire/GaN interface 304 absorbs the photons very strongly, resulting in localized heating. This localized heating is sufficient to disrupt the Gaxe2x80x94N bonds, breaking the strained but coherent interface between the lattice of the substrate 301 and the film 302. Typically, the laser beam is swept across the backside of the wafer to gradually release the epitaxial film from the substrate. If the beam is sufficiently intense, all bonds will be broken, isolating the two lattices. A less intense beam may be used to partially disrupt the interface, breaking as few as 5% of the bonds, if such an effect is desired. In FIG. 3(c) the process has continued. If the laser beam is too intense or not swept properly, localized hot areas can develop where the pressure from liberated nitrogen gas beneath the epitaxial film can build up and cause a rupture in the surface of the film, 305. Additionally, residual thermal stresses in the as-yet unreleased areas can cause cracks 306 to develop in the film, especially as the stress profile changes during the debonding process. Both of these effects are undesirable and must be avoided, typically by careful modulation of the impinging laser power and scan rate, choosing a laser with a short pulse length, and/or using a beam homogenizer to form an illuminated spot with uniform intensity, among other techniques. Even with such precautions, cracking of the released epitaxial film may still occur, preventing the lift-off and removal of a whole layer to be used as a free-standing substrate.
FIG. 4 schematically represents a series of steps involved in the conventional method for laser lift-off of a GaN film from a sapphire substrate. In step 401 a prepared substrate is provided. This prepared substrate may be, for example, plain sapphire, chemically cleaned prior to use. In step 402, the substrate has a layer of GaN 302 deposited onto it at an elevated growth temperature. In step 403 the substrate with GaN epitaxy is allowed to cool to the ambient temperature and is unloaded from the growth apparatus. In step 404 the grown wafer is placed into the LLO apparatus, which typically consists of a laser, laser power regulator, a wafer holder, and a beam steering mechanism to allow the beam 303 to impinge over the entire backside surface of the wafer. The beam then impinges over the backside of the wafer, gradually debonding the epitaxial film from the sapphire. In step 405, the debonding is complete, the debonded epitaxial film is removed from the sapphire by heating the wafer above 30xc2x0 C. (the melting point of gallium metal) and the layers are gently pulled apart. Often, the debonded layer is cleaned in an acid solution to dissolve any remaining gallium from its backside surface. Although free-standing epitaxial GaN films may be produced by LLO, the high stress between the sapphire substrate and the GaN layer often leads to cracking, fractures and other failures in the GaN layer. Thus the yield of usable free-standing epitaxial GaN films is often unacceptably low.
Because of the problems encountered with growing thick layers of GaN on sapphire, and of the problems encountered in attempting to remove GaN from the sapphire substrate using a conventional LLO technique, a need exists for a method for the laser lift-off and removal of GaN films from sapphire substrates for the creation of high quality free-standing substrates.
The present invention provides a method for the production of crack-free Group III-Nitride layers. The method proceeds by growing a crack-free first layer of Group III-Nitride on a starting substrate. A partial to complete loss of coherency is then achieved between a lattice of the first layer and a lattice of the starting substrate. A second layer is grown to form a composite layer that includes the first layer and the second layer such that the first layer is between the second layer and the substrate.
The present invention also provides a method for the production of arbitrarily thick, crack-free, freestanding layers of GaN or similar material for subsequent use as substrates. This method proceeds by growing a crack-free first layer of Group III-Nitride on a starting substrate. A partial to complete loss of coherency between a lattice of the first layer and a lattice of the starting substrate is then achieved. A second layer is grown to form a composite layer that includes the first layer and the second layer, and where the first layer is between the second layer and the substrate. The starting substrate is then completely separated from the composite layer to produce the freestanding substrate. In both methods, an intense light source may be used to partially disrupt the interface between this layer and the underlying starting substrate, making said interface partially incoherent.
In both methods a crack-free second layer may be grown on top of a crack-free first layer that has a partially incoherent interface with respect to the underlying starting substrate.
Furthermore, a crack-free second layer may be grown on top of a crack-free first layer (which has a partially incoherent interface with respect to the underlying starting substrate), in-situ, without necessitating a further cooling-down step.
These and other objects, advantages, and features of the invention will be set forth in part in the description which follows, and in part will become apparent to those having ordinary skill in the art upon examination of the following, or may be realized and attained as particularly pointed out in the appended claims.